plate count statistics

Dr. David Rosen drosen
Wed Nov 22 13:33:18 EST 1995

Bio Techniques Laboratories <cobactin at halcyon.com> wrote:
>In recent years I have found two editorials on the "correct" method to 
>statistically analyze plate count data.  The first one said that the mean 
>and the std. dev. should be determined from the arithmetic values of the 
>data.  The second one said the data should be transformed to logs and 
>then the mean and std. dev. should be calculated and then retransformed 
>to scientific notation. 
	I am just guessing here, but it may depend on the growth phase
that you expect the bacteria to be in.  One wants to choose a procedure
to minimize the ratio of STD to mean.  Consider the S-curve.  If the
bacteria are in the top part of the curve, where the concentration of
bacteria is almost constant, take mean from arithmetic values.  If you
are sure most are in the log-normal phase, take mean from logs.
drosen at inxs.chem.duke.edu

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